A preparation of single cardiac cells from which the surface membrane has been removed by microdissection (skinned cardiac cells) has been developed by the applicants. The preparation permits the perfused solutions to come in direct contact with the structures that participate in the contraction and the excitation-contraction coupling of cardiac muscle. Previous work has indicated that the contractions of this preparation are activated by a Ca 2 ion-induced release of Ca 2 ion from the sarcoplasmic reticulum. During the third year of this grant, the properties of the myofilaments will be explored by direct tension recording with a transducer sensitive in the microgram range and by high speed cinematography of the sarcomere during contraction. It is proposed to study the effect of varying sarcomere length on the active tension induced either by decreasing the (MgATP) in the absence of free Ca 2 ion (rigor tension) or by a saturating free (Ca 2 ion) or by a suboptimal free (Ca 2 ion). The aim of this study is to test if the length-tension diagram of skinned cardiac cells can be explained by the cross-bridge theory. This study will be completed by the assessment of the effects of either slow (1 sec) or relatively rapid (0.1 sec) length changes of the contractile activation. Finally the sarcomere kinetics during relaxation will be explored.